Nowadays, electronic devices such as DVD players, PCs, and TVs have become absolutely necessary in people's life. However, an unexpected power break in such electronic devices occurs when power supplied to the electrical devices exceeds a threshold value. In order to protect electronic devices from damage by power break, an overpower-protection circuit is usually provided in an electronic device to shut the power down when it exceeds a threshold value. With the power's shutting down, the electronic devices are hung up and not resumed until they are restarted or plugged out/in the power supply. Such a power break not only probably damages the devices, but also scares the users. Especially, when the power break occurs, it might result in undesired data loss in the data processing device such as PCs.
FIG. 1A shows a circuit diagram of the power supply apparatus 1 having a conventional overpower-protection circuit 10.
As shown in FIG. 1A, the power supply apparatus 1 includes a rectifier 12 for rectifying the current from a power supply (AC POWER) 18. A switch transformer 14 is electrically coupled to the rectifier 12 to transform the rectified current from the rectifier 12 into a predetermined current, which is in turn applied to the load 20 via a load protection circuit 108 that is formed of a capacitor C1, a Zener diode ZD1 and resistors R1, R2, R3, and R5.
The power supply apparatus 1 is also equipped with a conventional overpower-protection circuit 10, including a photo coupler 104 and a DC-DC converter 102 providing a switching voltage for the operation of the switch transformer 14. The converter 102 has its first input electrically coupled with the rectifier 12 and second input (pin FB) 102a. The photo coupler 104 is formed of a LED 104a optically coupled to a transistor 104b. The transistor 104b has its collector electrically coupled with second input (pin FB) 102a of the converter 102 and its emitter be grounded.
The LED 104a has its anode electrically coupled with the load 20 via resistor R1 and its cathode electrically coupled with the load 20 too, via capacitor C1 and two resistors R2 and R3, as shown in FIG. 1A.
FIG. 1B shows the circuit of the power supply apparatus 1 in detail. As shown in FIG. 1B, the power supply (AC POWER) 18 has the voltage in a range of 85˜265V and the frequency of 50/60 Hz. The rectifier 12 includes a bridge of diodes BD, which rectifies the current from the power supply (AC POWER) 18 from AC current to DC current, and outputs the rectified current to the switch transformer 14 and to the DC-DC converter 102 via a pair of resistors (R101, R102) coupled in parallel. The switch transformer 14 includes transforming coils T101 for transforming the voltage corresponding to the rectified current from the rectifier 12 into a predetermined voltage, and applying the predetermined voltage to a load 20. In FIG. 1B, the input voltage and current of the load 20 are 12V and 3.5 A respectively. The DC-DC converter 102 is an IC chip of eight pins, whose model number is FAN7554, manufactured by Fairchild Semiconductor Corporation in U.S.A., and available in market since 2003.
FIG. 1C shows the circuit diagram of the FAN7554 as shown in FIG. 1B.
The FAN7554 has many built-in protection circuits that do not need additional components, providing reliability without cost increase. These protection circuits have the auto-restart configuration. In this configuration, the protection circuits reset when Vcc is below UVLO stop threshold (9V) and restarts when Vcc is above UVLO start threshold voltage (15V).
Abnormalities may occur in the SMPS secondary side feedback circuit. First, when the feedback pin is short to the ground, the feedback voltage is zero and the FAN7554 is unable to start switching. Second, when the feedback circuit is open, the secondary voltage generally becomes much greater than the rated voltage as the primary side continues to switch at the maximum current level. This may cause the blowing off the fuse or, in serious cases, fires. It is possible that the devices directly connected to the secondary output without a regulator could be destroyed. Even in these cases, the over voltage protection circuit operates. Since Vcc is proportional to the output, in an over voltage situation, it also will increase. In the FAN7554, the protection circuit operates when Vcc exceeds 34V. Therefore, in normal operation, Vcc must be set below 34V.
An overload is the state in which the load is operating normally but in excess of the preset load. The overload protection circuit can force the FAN7554 to stop its operation. The protection can also operate in transient states such as initial SMPS operation.
Because the transient state returns to the normal state after a fixed time, the protection circuit need not to operate during this time. That is, the FAN7554 needs the time to detect and decide whether it is an overload condition or not. The protection circuit can be prevented from operating during transient states by ensuring that a certain amount of time passes before the protection circuit operates. The above operations are executed as follows: Since the FAN7554 adopts a current mode, it is impossible for current to flow above a maximum level. For a fixed input voltage, this limits power. Therefore, if the power at the output exceeds this maximum, Vo, becomes less than the set voltage, and the KA431 pulls in only the given minimum current. As a result, the photo-coupler's secondary side current becomes zero. The same goes for the photo-coupler's primary side current. Consequently, when the full current 1 mA flows through the internal resistor (2R+R=3R), Vfb becomes approximately 3V and from that time, the 5 uA current source begins to charge Cfb, the photo-coupler's secondary current is almost zero. The FAN7554 shuts down when Vfb reaches 6V.
The pin definitions of the FAN 7554 are listed in the below table.
Pin NumberPin NamePin Function Description1FBInverting (−) input of pwm comparator,on/off control &  OLP sensing terminal.2S/SSoft start3ISNon-inverting (+) input of PWMcomparator, OCL sensing terminal4Rt/CtOscillator time constant(Rt/Ct)5GNDGround6OUTOutput of gate driver7VccPower supply8VrefOutput of 5 V reference
FIG. 2 is a schematic diagram showing the operational principle of power supply apparatus 1 having a conventional overpower-protection circuit 10, as shown in FIG. 1A.
As shown in FIG. 2, P1 represents an expected power and P2 a threshold power from power supply apparatus 1. When P1 does not exceed P2, voltage on the load 20 linearly varies as the current. However, when P1 exceeds P2, as shown in FIG. 2, the overpower-protection circuit 10 works to initiate the overpower protection, bringing the power on the load 20 to zero. This might not only scare the user but also damage the device and/or the data in processing.
To solve the problems, what is needed is an overpower-protection circuit and a power supply apparatus having the same, which can output voltage declines smoothly when current rises continuously, avoiding or eliminating the damage to the devices.